Paper webs or sheets, sometimes called tissue or paper tissue webs or sheets, find extensive use in modern society. These include such staple items as paper towels, facial tissues and sanitary (or toilet) tissues. These paper products can have various desirable properties, including wet and dry tensile strength, absorbency for aqueous fluids (e.g., wettability), low lint properties, desirable bulk, and softness. The particular challenge in papermaking has been to appropriately balance these various properties to provide superior tissue paper.
Although somewhat desirable for towel products, softness is a particularly important property for facial and toilet tissues. Softness is the tactile sensation perceived by the consumer who holds a particular paper product, rubs it across the skin, and crumples it within the hand. Such tactile perceivable softness can be characterized by, but is not limited to, friction, flexibility, and smoothness, as well as subjective descriptors, such as a feeling like velvet, silk or flannel. This tactile sensation is a combination of several physical properties, including the flexibility or stiffness of the sheet of paper, as well as the texture of the surface of the paper.
Stiffness of paper is typically affected by efforts to increase the dry and/or wet tensile strength of the web. Increases in dry tensile strength can be achieved either by mechanical processes to insure adequate formation of hydrogen bonding between the hydroxyl groups of adjacent papermaking fibers, or by the inclusion of certain dry strength additives. Wet strength is typically enhanced by the inclusion of certain wet strength resins, that, being typically cationic, are easily deposited on and retained by the anionic carboxyl groups of the papermaking fibers. However, the use of both mechanical and chemical means to improve dry and wet tensile strength can also result in stiffer, harsher feeling, less soft tissue papers.
Certain chemical additives, commonly referred to as debonding agents, can be added to papermaking fibers to interfere with the natural fiber-to-fiber bonding that occurs during sheet formation and drying, and thus lead to softer papers. These debonding agents are typically cationic and have certain disadvantages associated with their use in softening tissue papers. Some low molecular weight cationic debonding agents can cause excessive irritation upon contact with human skin. Higher molecular weight cationic debonding agents can be more difficult to apply at low levels to tissue paper, and also tend to have undesirable hydrophobic effects on the tissue paper, e.g., result in decreased absorbency and particularly wettability. Since these cationic debonding agents operate by disrupting interfiber bonding, they can also decrease tensile strength to such an extent that resins, latex, or other dry strength additives can be required to provide acceptable levels of tensile strength. These dry strength additives not only increase the cost of the tissue paper but can also have other, deleterious effects on tissue softness. In addition, many cationic debonding agents are not biodegradable, and therefore can adversely impact on environmental quality.
Mechanical pressing operations are typically applied to tissue paper webs to dewater them and/or increase their tensile strength. Mechanical pressing can occur over the entire area of the paper web, such as in the case of conventional felt-pressed paper. More preferably, dewatering is carried out in such a way that the paper is pattern densified. Pattern densified paper has certain densified areas of relatively high fiber density, as well as relatively low fiber density, high bulk areas. Such high bulk pattern densified papers are typically formed from a partially dried paper web that has densified areas imparted to it by a foraminous fabric having a patterned displacement of knuckles. See, for example, U.S. Pat. No. 3,301,746 (Sanford et al), issued Jan. 31, 1967; U.S. Pat. No. 3,994,771 (Morgan et al), issued Nov. 30, 1976; and U.S. Pat. No. 4,529,480 (Trokhan), issued Jul. 16, 1985.
Besides tensile strength and bulk, another advantage of such patterned densification processes is that ornamental patterns can be imprinted on the tissue paper. However, an inherent problem of patterned densification processes is that the fabric side of the tissue paper, i.e. the paper surface in contact with the foraminous fabric during papermaking, is sensed as rougher than the side not in contact with the fabric. This is due to the high bulk fields that form, in essence, protrusions outward from the surface of the paper. It is these protrusions that can impart a tactile sensation of roughness.
The softness of these compressed, and particularly patterned densified tissue papers, can be improved by treatment with various agents such as vegetable, animal or synthetic hydrocarbon oils, and especially polysiloxane materials typically referred to as silicone oils. See Column 1, lines 30-45 of U.S. Pat. No. 4,959,125 (Spendel), issued Sep. 25, 1990. These silicone oils impart a silky, soft feeling to the tissue paper. However, some silicone oils are hydrophobic and can adversely affect the surface wettability of the treated tissue paper, i.e. the treated tissue paper can float, thus causing disposal problems in sewer systems when flushed. Indeed, some silicone softened papers can require treatment with other surfactants to offset this reduction in wettability caused by the silicone. See U.S. Pat. No. 5,059,282 (Ampulski et al), issued Oct. 22, 1991.
Besides silicones, tissue paper has been treated with cationic, as well as noncationic, surfactants to enhance softness. See, for example, U.S. Pat. No. 4,959,125 (Spendel), issued Sep. 25, 1990; and U.S. Pat. No. 4,940,513 (Spendel), issued Jul. 10, 1990, that disclose processes for enhancing the softness of tissue paper by treating it with noncationic, preferably nonionic, surfactants. However, the '125 patent teaches that greater softness benefits are obtainable by the addition of the noncationic surfactants to the wet paper web; the '513 patent only discloses the addition of noncationic surfactants to a wet web. In such "wet web" methods of addition, the noncationic surfactant can potentially migrate to the interior of the paper web and completely coat the fibers. This can cause a variety of problems, including fiber debonding that leads to a reduction in tensile strength of the paper, as well as adverse affects on paper wettability if the noncationic surfactant is hydrophobic or not very hydrophilic.
Tissue paper has also been treated with softeners by "dry web" addition methods. One such method involves moving the dry paper across one face of a shaped block of wax-like softener that is then deposited on the paper surface by a rubbing action. See U.S. Pat. No. 3,305,392 (Britt), issued Feb. 21, 1967 (softeners include stearate soaps such as zinc stearate, stearic acid esters, stearyl alcohol, polyethylene glycols such as Carbowax, and polyethylene glycol esters of stearic and lauric acids). Another such method involves dipping the dry paper in a solution or emulsion containing the softening agent. See U.S. Pat. No. 3,296,065 (O'Brien et al), issued Jan. 3, 1967 (aliphatic esters of certain aliphatic or aromatic carboxylic acids as the softening agent). A potential problem of these prior "dry web" addition methods is that the softening agent can be applied less effectively, or in a manner that could potentially affect the absorbency of the tissue paper. Indeed, the '392 patent teaches as desirable modification with certain cationic materials to avoid the tendency of the softener to migrate. Application of softeners by either a rubbing action or by dipping the paper would also be difficult to adapt to commercial papermaking systems that run at high speeds. Furthermore, some of the softeners (e.g., the pyromellitate esters of the '065 patent), as well as some of the co-additives (e.g., dimethyl distearyl ammonium chloride of the '532 patent), taught to be useful in these prior "dry web" methods are not biodegradable.
Accordingly, it would be desirable to be able to soften tissue paper, in particular high bulk, pattern densified tissue papers, by a process that: (1) uses a "dry web" method for adding the softening agent; (2) can be carried out in a commercial papermaking system without significantly impacting on machine operability; (3) uses softeners that are nontoxic and biodegradable; and (4) can be carried out in a manner so as to maintain desirable tensile strength, absorbency and low lint properties of the tissue paper.